Mobile communications systems refer generally to any tele-communications system which enables a wireless communication when users are moving within the service area of the system. A typical mobile communications system is a public land mobile network (PLMN). Often the mobile communications network is an access network providing a user with a wireless access to external networks, hosts, or services offered by specific service providers.
TETRA (TErrestrial Trunked RAdio) is a standard defined by ETSI (European Telecommunications Standard Institute) for digital professional mobile radio or private mobile radio (PMR) systems. The TETRA system is developed primarily for professional and governmental users, such as police, military forces, oil plants, etc.
One of the main targets in the development of mobile communications networks is to provide new data communication services, such as packet data communication, and especially IP (Internet protocol) services. Therefore, a new TETRA packet data protocol (PDP) is under standardization in ETSI. The TETRA PDP will extend the conventional TETRA network to work as an IP subnet.
The TETRA PDP extends data communication services by providing increased capacity and usability for the TETRA. TETRA packet data is built on top of the basic TETRA radio link protocol stack and provides service mechanisms to convey different higher layer protocols. The network layer protocols supported by the TETRA PDP include Internet Protocol (IP) versions 4 and 6. Thus the TETRA packet data extends the TETRA network to act as an IP subnet in the mobile IP scheme, which enables application programmers to build their applications in a well standardized environment.
In the draft specifications there are defined only two reference points for the TETRA PDP. These reference points are between an IP packet mode equipment (TE) and a mobile termination (MT) and between MT and TETRA Switching and Management Infrastructure (SwMI), respectively. In other words, the draft specifications of ETSI define how TETRA PDP operates from the point of view of the air interface and how to connect an IP packet-mode terminal equipment TE (e.g. PC or laptop) to a TETRA mobile terminal MT. No reference points inside the SwMI nor between the SwMI and an external IP network have been defined. In other words, the draft ETSI specifications do not define how to connect the TETRA PDP to the external IP networks which accommodate the IP services the user is interested in. As a consequence, the internal structure of the SwMI and its interworking with external IP networks is dependent on implementation.
In this context the interconnectivity of the TETRA PDP can be divided into three parts: 1) Interworking with external IP Networks, e.g. the Internet and private intranets; 2) Interworking between separate TETRA networks; and 3) Interworking between the TETRA PDP and other mobile communications networks.
In the interworking between the TETRA PDP and external IP networks, users' requirements for the TETRA PDP may vary a lot. The basic need is naturally the same: a user wants to use IP-based applications located on an external IP network, either on the Internet or on an intranet. From the user's perspective a network which resides between a user terminal and application services is merely a transport media for IP datagrams. However, even if the basic functionality is the same, the way the connection is established and packets are routed from one network to another varies.
When the PDP context is established (i.e. the subscriber's IP address is activated and data can be transferred), the following issues may vary among users: 1) Allocation of the IP address (e.g. whether a static or dynamic allocation is used, and whether the IP address is allocated from the operator's or customer's address space); 2) User authentication (e.g. the authentication method used, whether the authentication is carried out by the operator and/or the customer); 3) User authorization; 4) Encryption of the IP traffic (none, end-to-end encryption, encryption between inteworking networks); and 5) Tunneling of the traffic.
The routing needs may also be very different for different users:
i) All IP packets from the mobile stations MS of a TETRA customer are routed to the customer's intranet, regardless of the IP destination address in the packets. In this case the customer's intranet provides all the IP services which the users want to use, for example access to the Internet is provided through the customer's own network. Possible solutions include tunneling protocols, such as the GRE, L2TP and IPSEC tunnel mode. Specific IP routing techniques may also be used, such as MPLS.
ii) All IP packets from the mobile station of the TETRA customer are routed to the Internet through the TETRA operator's Internet gateway. In this case there is no direct connection from the TETRA operators's network to its customer's intranet but all IP packets from the MSs are routed to the Internet. This means that the TETRA operator works as an Internet Service Provider (ISP).
iii) The TETRA operator provides IP-based services on their own IP network. In this case the operator has IP services on their own network and provides them for the TETRA customers. Such services could include e-mail, WAP services and services tailored for the customers.
iv) The remaining needs may be combinations of the basic schemes described above. In this case the TETRA operator's customer uses services provided on several IP networks. For example, there are important services on the customer's own intranet, the TETRA operator provides some specific IP services on its own network, and Internet access is routed through the TETRA operator's Internet gateway.
The IP interworking between TETRA networks should provide the TETRA subscribers with the possibility to use TETRA PDP services in multiple SwMIs, i.e. TETRA networks. For example, a user should be able to activate the PDP context for IP traffic in a home SwMI and then visit a foreign SwMI and continue using IP services without any additional arrangements.
At present there are no implemented solutions fulfilling the interworking needs described above.